2016-12-09T13:40:14ZAplicação de ligas com memória de forma para rebiteshttp://hdl.handle.net/10362/19362
Título: Aplicação de ligas com memória de forma para rebites
Autor: Camacho, Edgar Alexandre Freitas
Resumo: A presente dissertação tem como objetivo estudar a possibilidade de utilização de ligas com memória de forma (LMF) (NiTi) em processos de união de chapas, nomeadamente por rebitagem.
O efeito de memória de forma (EMF) pode ser utilizado para realizar processos de união de diversos componentes. No entanto, não há indicações de uma utilização extensa deste tipo de materiais para esse fim. Uma patente recente (2013) no domínio da aeronáutica abre perspetivas interessantes para este tipo de processos de união.
No estudo de conceito e viabilidade de um rebite com memória de forma, executaram-se diversos tipos de ensaios: calorimetria diferencial de varrimento (DSC), compressão, dilatometria, e ensaios termomecânicos. Os provetes estudados foram sujeitos a tratamentos térmicos de recristalização, para estudar a possibilidade de reutilização deste tipo de materiais. Concluídos estes ensaios e respetiva caracterização, demonstrou-se a viabilidade da utilização de rebites com memória de forma. Para esse efeito, foi criado um protótipo de teste, que quando ensaiado atingiu um valor máximo de 340 N, para separar duas chapas ligadas por este material.
A utilização deste tipo de material para o processo de rebitagem é viável, contudo, apenas quando aplicado em materiais com uma tensão de cedência inferior à da tensão exercida por EMF. Respeitados estes constrangimentos, para além de promover o encruamento das chapas a unir, promove-se a deformação também do elemento de união (LMF) e o consequente aumento da histerese térmica do rebite.2016-03-01T00:00:00ZNanoparticles as a charge trapping layer in Metal-Insulator-Semiconductor structureshttp://hdl.handle.net/10362/19276
Título: Nanoparticles as a charge trapping layer in Metal-Insulator-Semiconductor structures
Autor: Carreiras, Rui Filipe Raposo
Resumo: Memories with floating gate structures are the main device architecture used in current non-volatile memories. Different films for floating gate based devices have been studied to substitute poly-crystalline silicon as the main material in floating gate structures. In current technology tunneling oxides are required to have thicknesses around 30 nm reducing device performance. Nanocrystals and nanoparticles have been emerging as a possible replacement for those films since better retention times and faster devices can be obtained. In this work the study of nanoparticles as the Charge trapping layer was executed. Study of the nanoparticles was made in a MIS structure. Hysteresis loops on C-V curves showing charge trapping was expected. Molybdenum film in the charge trapping layer was characterized as a comparison term for the nanoparticles in the CT-layer. Results of this work detail the importance of the interfacial layers, as well as defects across the oxides, on the electrical characterization of this structures. Hole trapping was achieved with nanoparticles as a charge trapping layer. Data obtained demonstrated the effect of interfacial defects in C-V curves as well as charging behavior in gold nanoparticles and Molybdenum films.2016-03-01T00:00:00ZStudy on the viability of a 4x2 HEB mixer array at super-THz based on a Fourier phase grating LO for space applicationshttp://hdl.handle.net/10362/19275
Título: Study on the viability of a 4x2 HEB mixer array at super-THz based on a Fourier phase grating LO for space applications
Autor: Silva, José Rui Gaspar da
Resumo: Various astronomical telescopes including Herschel, ALMA, STO and SOFIA-GREAT have successfully exploited THz radiation in order to explore the cosmos from the birth of the Universe to the life cycle of individual stars. The THz region, however, still remains relatively poorly observed due to poor transmission of THz light through Earth’s atmosphere as a result of past technological constraints. One such example is the observation of neutral oxygen [OI] at 4.7 THz which is only now opening up as a possibility for astronomical study.
In this thesis we report on technology development aimed at implementing an advanced 4.7 THz astronomical receiver array for a proposed future NASA balloon mission called GUSTO. Several aspects of the receiver are studied in detail: HEB characterization and selection; Impact of lens size on device sensitivity; LO multiplexing at 1.4 THz using a 4x2 Fourier phase grating as a stepping stone to a 4.7 THz Fourier grating, including a demonstration of a 2x2 pixel array receiver using the central four beams of the grating output beam pattern. Conclusions are presented on the findings that will have direct input to the GUSTO mission.2016-09-01T00:00:00ZConductive bridging RAM devices inspired on solid-state biopolymer electrolyteshttp://hdl.handle.net/10362/19191
Título: Conductive bridging RAM devices inspired on solid-state biopolymer electrolytes
Autor: Freitas, Pedro Nuno de Jesus Francisco
Resumo: This work reports the design, fabrication and characterization of metal-insulator-metal (MIM) structures acting as conductive bridging random access memory (CBRAM) devices using biopolymer insulator. Chitosan and hydroxypropyl cellulose (HPC) were deposited by spin coating in between evaporated Pt and Ag electrodes. CBRAM devices fabricated using chitosan as the insulating layer demonstrated retention times of up to 105 s with an on/off ratio of 102 as well as enduring several program/erase cycles. Devices fabricated with HPC showed retention times of up to 104 s with an on/off ratio of approximately 106, and also showed stable device operation over several cycles.
Furthermore, the functionalization of chitosan with silver nanoparticles and its integration in the MIM structures were investigated, as well as the substitution of the e-beam evaporated Ag electrode by a screen printed Ag electrode.2016-03-01T00:00:00Z